The present invention relates to a developing device for use in an electrophotographic copying apparatus, a printer or the like.
In response to a recent keen demand for more compact and more inexpensive copying apparatuses, printers, etc., there is a tendency that developing devices of a type employing non-magnetic toner as one-component developer are utilized in the copying apparatuses, the printers, etc. The developing devices of this kind are provided with a toner support member having a movable surface for supporting toner and a regulation member for electrically charging the toner supplied onto the toner support member and for regulating amount of the toner adhering to the toner support member. In such developing devices, it is essential to form a properly charged toner layer of a proper thickness on the toner support member.
As shown in FIG. 1, a typical example of the known regulation member is a blade 2 which is formed by an elastic plate and is pressed against a toner support member 1. However, in case the blade 2 is employed, amount of the toner adhering to the toner support member 1 varies according to a dimension l of a projecting portion of the blade 2. This projecting portion of the blade 2 extends from a point P of contact between the blade 2 and the toner support member 1 to a distal end of the blade 2.
FIG. 2 shows relation between a length L of the blade 2 and amount M (mg/cm.sup.2) of the toner adhering to the toner support member 1 in the case where the blade 2 is pressed against the toner support member 1 of a sleeve type shown in FIG. 1. In order to obtain a practical thin layer of the toner on the toner support member 1 shown in FIG. 1, the amount M of the toner adhering to the toner support member 1 should range from 0.4 to 0.5 mg/cm.sup.2. If the dimension l of the projecting portion of the blade 2 for securing the practical amount M of the toner adhering to the toner support member 1 is determined in FIG. 2, the dimension l of the projecting portion of the blade 2 should range from 0 to 0.5 mm corresponding to the length L of 22.0 to 22.5 mm. However, such a problem arises that the developing device is required to be assembled with extremely high precision in order to obtain the dimension l of the projection portion of the blade 2 accurately.
In order to solve this problem, Japanese Patent Laid-Open Publication (unexamined) No. 46577/1985, for example, proposes a method shown in FIG. 3 in which a curved portion 3 having a circular cross section is provided on the regulation member so as to be brought into contact with the toner support member 1. Such curved portion 3 has advantages that (1) a permissible setting width of the regulation member is increased and (2) since a motor for driving the toner support member can be made more compact due to reduction in a mechanical torque for rotating the toner support member, the developing device itself can be made more compact and can be manufactured at a lower cost.
The advantage (1) is gained as follows. Namely, if the curved portion 3 is employed, the dimension of the projecting portion of the blade becomes approximately identical with a radius r of curvature of the curved portion 3. Thus, if only the radius r of curvature of the curved portion 3 is obtained accurately, the dimension of projecting portion of the blade is kept substantially constant at the radius r of curvature. Therefore, even if the blade is set at slightly different locations on the toner support member 1 as shown in FIGS. 4a, 4b and 4c during assembly of the developing device, it becomes possible to form a stable thin layer of the toner on the toner support member at all times.
Meanwhile, the advantage (2) is described as follows. If the curved portion 3 is brought into contact with the toner support member 1 as shown in FIG. 5a, a nip (contact) width W' of the curved portion 3 becomes smaller than a nip width W of the flat blade 2 as shown in FIG. 5b. Therefore, when an identical thin layer of the toner is formed on the toner support member 1 in both cases of FIGS. 5a and 5b, a contact pressure of the curved portion 3 and the blade 2 per unit area of the toner support member 1 is required to be made equal to each other. It is, needless to say, apparent that the torque of the motor for driving the toner support member 1 in the case of the curved portion 3 having the nip width W' narrower than the nip width W of the flat blade 2 is advantageously smaller than that of the flat blade 2.
However, even in the case where the regulation member having the curved portion is employed, quantity of electric charge of the toner on the toner support member and amount of the toner adhering to the toner support member vary according to the radius of curvature of the curved portion and a radius of curvature of the toner support member held in contact with the curved portion and thus, finally obtained image quality may be adversely affected.